The present invention evolved from continuing development efforts pertaining to subject matter disclosed in my co-pending Application Ser. No. 265,128, filed May 19, 1981, entitled "Symmetric Microwave Mixer". This co-pending Application discloses a diode mixer with compact, symmetric conductor port structure significantly reducing unwanted sensitivity to adverse perameters. One of the features afforded by the substrate circuit layout is the keyed alignment mounting of the diode quad in a singular precise position positively locking the quad in place. This singular registry is facilitated by two of the leads from the diode quad extending through the substrate.
The present invention relates to improvements in the microstrip circuit layout and structure in those applications where coplanar diode connection is desired. Such applications arise where other factors, e.g., cost, outweigh the type of mounting above described.
In a particularly desirable aspect of the present invention, coplanar diode mounting is achieved without sacrificing compactness or symmetry of the mixer port structure. The mixer port structure of the present invention significantly reduces unwanted sensitivity to adverse parameters, and also overcomes problems of distortion caused by unequal conduction paths and diode lead lengths. Transmission lengths along the conductors leading to the diode quad mixer are minimized, to reduce loss and exposure to mismatch influences. Transmission path lengths through the diodes of the mixer quad are also minimized, and are substantially equal, to thus eliminate unwanted phase shift due to differential path lengths through the mixer diodes. The diode mixer leads are short and orthogonal, to provide good impedance matching and minimize unwanted coupling.
In a further desirable aspect of the invention, a single mixer of the diode quad type is provided and affords microwave frequency conversion with desirable microstrip transmission characteristics, as opposed to various undesirable slotline or coplanar waveguide transmission characteristics and the like and their attendant constraints. Image frequency signals generates in the two diode pairs circulate in the diode quad only, to return the image frequency power to the diodes to reduce conversion loss. There is mutual isolation between RF, LO and IF signals over a broad frequency range. The circuit accepts transposed RF and LO inputs.
The operating bandwidth of the mixer is significantly increased by the use of the balanced port structure of the invention, the equalized conduction path lengths, the short lead lengths, and single point connections between the diode pairs facilitating "zero lead length" between the diode connection points.
In another desirable aspect of the invention, standard printed circuit board techniques may be used for fabrication, resulting in significant cost reduction. The invention also enables the use of low dielectric constant substrate material, further reducing cost.
In a further aspect of the invention, microstrip circuit layout and structure is provided in double sideband applications affording coplanar mounting of a diode pair.